Suspensory behaviors and the neck: a comparative analysis of the cervical vertebrae of extant primates

As the interface between the head and trunk, the cervical spine performs a range of functions, including directing head movement and withstanding the forces of gravity and soft-tissue loading associated with the pectoral girdle. Morphological variation in the pectoral girdle’s bony components (scapula and clavicle) and the nuchal musculature has been linked to differences in locomotor behavior among extant primates, particularly between suspensory and nonsuspensory taxa. However, the degree to which these morphological differences are reflected in cervical vertebral morphology has yet to be examined. The goal of this study was to test biomechanical models of the cervical spine by comparing extant suspensory and nonsuspensory primates. Vertebral features were quantified using linear and three-dimensional coordinate data in 525 specimens representing 31 primate species. Relationships between vertebral morphology and locomotor behavior were analyzed using pairwise comparisons and phylogenetic comparative methods. Results support a functional link between cervical morphology and positional behavior in some features. Specifically, spinous process length and cross-sectional area are greater in suspensory primates when compared to nonsuspensory taxa. A number of features appear to mimic lumbar patterns of morphological adaptation to posture previously documented among primates, such as reduced craniocaudal height of the vertebral bodies in more orthograde taxa. The cervical spine thus presents locomotor- and posture-related functional signals, suggesting that this anatomical region can aid in inferring positional behaviors in fossil taxa.